This invention relates to rotary rock bits, and more specifically to improved manufacturing and bearing structure for the rotary cutter cones of the drill bit.
Rotary type drilling bits, such as are used in drilling rock for oil, utilize cutters which are subject to very high vertical loads from the weight of the drill string. The bearings for rotatably supporting the cutters are subjected to severe loading conditions which may cause the drill bit to fail even before the cutters wear out. The friction type journal bearing is superior over anit-friction ball or roller type bearings for withstanding high radial loads, but friction type bearings require continuous lubrication to prevent excessive wear and premature failure. This has led to the development of rock bits which include a lubricant reservoir which is in communication with the bearing area between the rotary cutter cone and the journal bearing shaft. Such a lubrication system is described, for example, in U.S. Pat. No. 3,917,028 which shows a lubrication system utilizing a reservoir which is pressure-compensated. In such a system with a pressure compensating reservoir, a lubricant passage leads from the compensator to the surface of a journal bearing on the shaft. A flat is often cut on the upper, unloaded side of the journal bearing where the lubricant passage intersects it. Use of a pressure compensator permits the lubrication system to be completely sealed against drilling mud and other contaminating materials while permitting the lubrication system to adjust to changes in pressure due to expansion and volatilization of the lubricant in response to the elevated temperatures and pressures to which the lubricant is subjected during operation.
To further improve wear of the journal bearing, it has been the practice for several years to provide a bearing metal on the journal leg or shaft which extends around the lower third of the circumference of the shaft in the region of maximum load. During drilling this fraction of the bearing is loaded and the balance has only light, if any, loads. An arcuate slot was milled in the bearing surface of the lower third of the shaft and filled with hard bearing metal by weld deposition. The surface of the bearing metal was then ground to a radius about 0.002 inch larger than the journal bearing surface and with an axis of curvature about 0.002 inch lower than the axis of the journal. Thus, the lower part of the bearing surface on the journal is offset downwardly a small amount. This eccentricity minimizes excessive squeeze on the O-ring seal on the loaded lower side of the journal and does not require an eccentric O-ring groove for the same purpose. Milling of the arcuate slot for the bearing metal necessitates a separate machining operation and does add to the cost of manufacture.
In U.S. Pat. No. 3,746,405 there is shown a rock bit in which an arcuate slot is milled in the upper unloaded half of the journal shaft, the lower half being case hardened. The space formed by the slot and surrounding sleeve is used to store a small quantity of lubricating grease. However, this arrangement requires a separate milling operation to form a slot around only a part of the periphery of the shaft. Further, the quantity of grease that can be stored is very limited and is not compensated for the effects of large pressure and temperature variations.